Re: Peter Hewing's shielding query

[Michael Williamson <m.williamson@mailbox.uq.edu.au>]

Peter Hewing wrote:

>
>I would like to determine the dose reduction of a wall of one of our
>buildings. I would like to do this by installing a radiation source (Co-60
>or something else, approx. 1 to 10 Ci) and measure the dose rate inside and
>outside the wall. I was told that there are a lot of ways to get the wrong
>values. Is there somebody who can point me on the right way to determine the
>dose reduction? Are there any reports, web-sites or articles on my point
>available?
>

Here are few initial points which some of the other radsafe folks may wish
to expand on:

The transmission value of the wall will vary with the energy of the
incident radiation.  If you use Co60 the value you derive will only be
appropriate for radiation with photon energies of the order of 1 MeV (Co60
gamma rays of energies of 1.117 and 1.335 MeV).  Lower energies will be
more heavily attenuated - how heavily will depend on the effective atomic
number of the shielding material.  So the first point is to select, if
possible, a test source of approximately the same energy as the source to
be shielded.

The second consideration is the geometry of the measuring set-up.  It is
very easy (especially at MeV energies) to obtain misleading results because
of scatter off the floors and ceiling as well as skyshine (air scatter).
You should try to duplicate the geometry of the actual source/shield
relationship as nearly as you can.  This way, you'll get a realistic idea
of the actual effect of the shield and be able to predict radiation levels
with some accuracy.  A good reference for design work of this kind is NCRP
51: Radiation Protection Design Guidelines for 0.1 -100 MeV Particle
Accelerator Facilities.

Thirdly, if you're going to be using a multi curie Co60 source you'll need
to take considerable safety precautions, both with regard to access control
for other employees and members of the public, as well as for the people
operating the exposure device and taking the measurements.  Naturally, you
would be well advised to hire approved industrial radiography equipment
(and experienced operators) rather than use some jury-rigged or home made
exposure device.  the operators will need to be in radio contact with the
people doing the measurements to avoid unnecessary exposure. It will
generally be better to have multiple meauring devices rather than have one
person wandering around trying to take measaurements and write them down at
the same time (although you will need to search for hot spots).

If you need to get approval from company management or the authorities to
carry out the tests you should prepare a risk assessment and a safe
working/emergency procedure.  This should address all the hazard issues and
detail the procedures you will adopt to control them.

Any further ideas from the rest of the list?

Mike




Michael Williamson				ph +61 7 3365 4504
University Radiation Protection Adviser		fax +61 7 3365 1577
The University of Queensland			m.williamson@mailbox.uq.edu.au
Brisbane 4072
Australia

Any advice or information given in this posting is the opinion of the
writer alone and other than where explicitly stated, does not represent the
views or policies of the University.


************************************************************************
The RADSAFE Frequently Asked Questions list, archives and subscription
information can be accessed at http://www.ehs.uiuc.edu/~rad/radsafe.html